When she was young, Manya Benenson's dad told her a story of two frogs that fall into a bucket of cream and swim around and around. The first one gives up and drowns, the second keeps going until he finds his struggles have churned the cream to butter, and he climbs out. As a fable, she said, it could sum up the movement that the late Peter Benenson began in the Observer 50 years ago this weekend.

It was a day for sentiment and inspirational stories yesterday, as Amnesty International celebrated its birthday with an event at St Martins in the Fields in central London. The celebration was held at the same Trafalgar Square church where Benenson, a bowler-hatted barrister, slipped away from work in 1961 and sat alone to dream up what has become the world's most renowned human rights organisation.

He had been enraged by reading a newspaper account of the arrest in Portugal of two students, whose crime had been to raise a toast to freedom. Benenson died in 2005 and yesterday his daughter Manya, 35, lit the Amnesty candle, symbolically ringed by barbed wire, in his memory, along with Wai Hnin Pwint Thon, a Burmese refugee whose father is serving a 65-year jail sentence for organising peaceful protests against the military junta in 2007.

"I don't think I will see my dad again," said after the ceremony. "But he always told me that as long as this regime is there, there is no happy ending for my family. My ten-year-old sister is still in Burma with my mother and I hope that one day she will have the chance of a better life and not have to leave. That hope is what Amnesty means to me."

At yesterday's event, the toasts were to freedom, and the guest list was global. Former prisoners of conscience, activists and supporters, including the children's author Michael Morpurgo and the actors Eva Birthistle, Tim McInnerny and Julian Rhind-Tutt, sat beneath banners illustrated with the faces of some of the 50,000 prisoners of conscience whose ordeals in the jails and torture rooms of corrupt regimes have been highlighted by Amnesty and its three million members. In red pen, scrawled across one banners, that of Eynulla Fatullayev, were the buoyant words "just released". On Wednesday, a day after a mass "tweet" protest, Fatullayev, the editor of a newspaper in Azerbijan who had been jailed for criticising government policies, was freed on a presidential pardon. He immediately thanked Amnesty, which supported him from the beginning, saying: "In my opinion you saved me. Thank you to all those who tweeted."

Salil Shetty, the director of Amnesty, said in his speech that Benenson was perhaps the first "Yes-we-can man", who saw that the abuse of state power could be tackled by a social movement that has now embraced tweeting and Facebook. "Social media re-energises the idea of the global citizen," he said, adding that when, on May 28, 1961, Benenson wrote in the Observer about the fate of "forgotten prisoners", it was a direct appeal to individuals to take action.

"It was only supposed to be a year-long campaign," said one of Amnesty's longest-term supporters, Dan Jones, 71. Aged 21, he accompanied his father, one of the three MPs Benenson enlisted to help launch Amnesty, to that first meeting 50 years ago today, and now runs his local Amnesty group around his own kitchen table. "Benenson didn't hang around and the day after he had the idea, he had the first meeting. Always the diplomat, he made sure he had a Tory, a Liberal, and my dad was the Labour MP. I remember thinking it was going to be a bit ambitious to keep the appeal going for a year!" The director of Amnesty International UK, Kate Allen, spoke and defended the movement against recent criticisms that it had been moving too far from its core business of supporting the individual "prisoner of conscience". "It would be wrong to think we no longer care about individuals," she said, insisting it remained about "ordinary people united in action, standing in solidarity".

For Manya Benenson, the day was a chance to show her daughters, Ami and Daisy, what their grandfather achieved. "I wish he could have been here, of course. I don't think he could ever have expected he could galvanise so many," she said.

To celebrate Amnesty International's 50th anniversary, the Guardian and the Observer have started a new online series. Every month we will publish news of an 'urgent action'; that is a current case of human rights abuse that Amnesty would like to draw wider attention to